Investments and Fair Value Measurements (Schedule Of Financial Assets Carried At Fair Value) (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 27, 2019		Sep. 28, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash Equivalents, at Carrying Value		$ 797,210
Cash				$ 918,064
Short-term investments, cost		413,927		380,795
Short-term investments, unrealized losses		0		(1)
Short term investments, total		416,210		377,723
Long-term investments, unrealized gains		0
Total cash, cash equivalents, and investments, cost		1,093,659		1,286,810
Total cash, cash equivalents, and investments, unrealized gains		2,341		343
Total cash, cash equivalents, and investments, unrealized losses		(57)		(3,170)
Total cash, cash equivalents, and investments, total		1,095,943		1,283,983
Assets		797,210		918,063
Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash				2,495
Short-term investments, cost		653
Short term investments, total		653
Long-term investments, unrealized gains		0
Long-term investments, unrealized losses		0
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash				5,058
Total cash, cash equivalents, and investments, cost		0		5,737
Total cash, cash equivalents, and investments, unrealized gains		0		1
Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash				1,005
Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash				544
Total cash, cash equivalents, and investments, cost		9,552		41,058
Other long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		1,700		400
Cost Method Investment, Fair Value Measurement Adjustment				200
Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost				178,838
Short-term investments, unrealized gains				64
Short-term investments, unrealized losses				(764)
Short term investments, total				178,138
Long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost				189,908
Long-term investments, unrealized gains				279
Long-term investments, unrealized losses				(2,405)
U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, unrealized gains		8
Short-term investments, unrealized losses		(9)
Short term investments, total		10,972
Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash Equivalents, at Carrying Value		680,287
Cash		680,287		905,660
Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash Equivalents, at Carrying Value		1,000
Cash		1,000
Short-term investments, unrealized losses		0
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash Equivalents, at Carrying Value		115,270
Cash		115,270		3,301
Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash Equivalents, at Carrying Value				545
Short-term investments, unrealized losses				(1)
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		796,210		911,456
Assets		821,664		935,306
Currency derivatives as hedge instruments, assets		0
Currency derivatives as hedge instruments, liabilities		0
Level 1 | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		653		2,495
Level 1 | Investments held in supplemental retirement plan:
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets		3,564		3,486
Liabilities		3,564		3,486
Level 1 | Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		680,287		905,660
Level 1 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		115,270		3,301
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		1,000		6,607
Assets		272,548		348,322
Currency derivatives as hedge instruments, assets		0
Currency derivatives as hedge instruments, liabilities		(242)
Level 2 | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		0
Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash				5,058
Level 2 | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		1,000		1,005
Level 2 | Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash				544
Level 2 | U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		10,972
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets		0		0
Currency derivatives as hedge instruments, assets		0
Currency derivatives as hedge instruments, liabilities		0
Short-term investments | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		8,381		7,970
Short-term investments, unrealized gains		11		0
Short-term investments, unrealized losses		(1)		(15)
Short term investments, total		8,391		7,955
Short-term investments | U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost				11,997
Short-term investments | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		6,347		4,276
Short-term investments, unrealized gains		9		0
Short-term investments, unrealized losses		0		0
Short term investments, total		6,356		4,276
Short-term investments | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		76,802		111,245
Short-term investments, unrealized gains		172		50
Short-term investments, unrealized losses		(34)		(494)
Short term investments, total		76,940		110,801
Short-term investments | Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		15,210		30,475
Short-term investments, unrealized gains		18		0
Short-term investments, unrealized losses		(7)		(120)
Short term investments, total		15,221		30,355
Short-term investments | Short-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		118,978
Short-term investments, unrealized gains		219
Short-term investments, unrealized losses		(51)
Short term investments, total		119,146
Short-term investments | Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		1,265		12,875
Short-term investments, unrealized gains		1		14
Short-term investments, unrealized losses		0		0
Short term investments, total		1,266		12,889
Short-term investments | U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		10,973
Short-term investments, unrealized gains				0
Short-term investments, unrealized losses				(135)
Short term investments, total				11,862
Short-term investments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		5,784		7,955
Short-term investments | Level 1 | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		5,784		7,955
Short-term investments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		113,362		170,183
Short-term investments | Level 2 | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		2,607
Short-term investments | Level 2 | Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		6,356		4,276
Short-term investments | Level 2 | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		76,940		110,801
Short-term investments | Level 2 | Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		15,221		30,355
Short-term investments | Level 2 | Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments		1,266		12,889
Short-term investments | Level 2 | U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments				11,862
Long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, total				187,782
Long-term investments | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost		23,563		15,966
Long-term investments, unrealized gains		187		0
Long-term investments, unrealized losses		0		(317)
Long-term investments, total		23,750		15,649
Long-term investments | Asset backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost		400
Long-term investments, unrealized gains		2
Long-term investments, unrealized losses		0
Long-term investments, total		402
Long-term investments | U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost		7,102		9,791
Long-term investments, unrealized gains		146		0
Long-term investments, unrealized losses		0		(166)
Long-term investments, total		7,248		9,625
Long-term investments | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost		134,360		146,561
Long-term investments, unrealized gains		1,700		33
Long-term investments, unrealized losses		0		(1,810)
Long-term investments, total		136,060		144,784
Long-term investments | Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost		10,315		17,235
Long-term investments, unrealized gains		87		0
Long-term investments, unrealized losses		(6)		(112)
Long-term investments, total		10,396		17,123
Long-term investments | Other long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost		1,731	[1]	355	[2]
Long-term investments, unrealized gains		0	[1]	246	[2]
Long-term investments, unrealized losses		0	[1]	0	[2]
Long-term investments, total		1,731	[1]	601	[2]
Long-term investments | Long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, cost		177,471
Long-term investments, unrealized gains		2,122
Long-term investments, unrealized losses		(6)
Long-term investments, total		179,587
Long-term investments | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		19,670		15,895
Long-term investments | Level 1 | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		19,670		15,649
Long-term investments | Level 1 | Other long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments	[2]			246
Long-term investments | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		158,186		171,532
Long-term investments | Level 2 | Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		4,080
Long-term investments | Level 2 | Asset backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		402
Long-term investments | Level 2 | U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		7,248		9,625
Long-term investments | Level 2 | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		136,060		144,784
Long-term investments | Level 2 | Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments		10,396		$ 17,123
Liabilities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Currency derivatives as hedge instruments, unrealized gains		0
Currency derivatives as hedge instruments, unrealized loss		(242)
Currency derivatives as hedge instruments, liabilities, cost		0
Currency derivatives as hedge instruments, liabilities		(242)
Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Currency derivatives as hedge instruments, assets, cost		0
Currency derivatives as hedge instruments, unrealized gains		0
Currency derivatives as hedge instruments, unrealized loss		0
Currency derivatives as hedge instruments, assets		$ 0

[1] Other long-term investments as of September 27, 2019 includes an investment that is not carried at fair value including an equity method investment of $1.7 million.

[2] Other long-term investments as of September 28, 2018 include a marketable equity security of $0.2 million, and other investments that are not carried at fair value including an equity method investment of $0.4 million. During fiscal 2018, we recorded write-off charges to reduce the carrying value of two cost method equity investments to zero in recognition of an other-than-temporary impairment for each investment.